1. Field of the Invention
This invention concerns a method of manufacturing a titanium magnetic disk substrate with excellent surface properties.
2. Description of the Related Art
Magnetic disk, used as a recording medium for a computer comprises a substrate and a magnetic film formed on the substrate. The substrate is required to have the following properties:
(a) Its surface properties after precision machining or precision grinding or precision polishing must be satisfactory so that magnetic heads can run smoothly over the disk, and such that stable magnetic properties are obtained with few magnetic errors.
(b) There must be no pertuberants or holes in its surface which could lead to defects in the magnetic film formed on the substrate.
(c) It should have sufficient strength and rigidity to withstand high speed rotation when the disk is used, and precision machining during manufacture.
(d) It must be able to withstand the heat applied when forming the magnetic film.
(e) It must be lightweight and non-magnetic.
(f) It must have some degree of corrosion resistance.
Conventionally, Al-Mg or other aluminium alloys are used as substrate materials satisfying these conditions.
Recently, there has been a trend in magnetic disks toward higher recording densities and greater compactness, so the following conditions must also be met:
(A) A magnetic film which permits high recording densities is formed by sputtering, so the substrate must have sufficient heat resistance to withstand the temperature rise produced in this process.
(B) To keep spacing losses to the absolute minimum and achieve high recording densities, the fling height of the magnetic heads is reduced, so the substrate surface must be very flat and smooth in order that the magnetic heads can move smoothly over it.
Aluminium alloys such as Al-Mg alloy which were conventionally used as substrate materials however had insufficient heat resistance, and they cannot be expected to achieve higher performance.
Various magnetic disk substrates using titanium have therefore been proposed in place of the conventional aluminium alloys.
Published Unexamined Japanese Patent Application No. 52-105804, for example, discloses a magnetic disk substrate of titanium or titanium alloy with an oxide or nitride film formed by oxidizing or nitriding of the surface; Published Unexamined Japanese Patent application No. 63-142521 discloses a magnetic disk substrate comprising a core layer of titanium or titanium alloy, an insert layer, a layer of nickel, titanium, nickel alloy or titanium alloy, and a layer of glass or ceramics superimposed in that order; and Published Unexamined Japanese Patent Application No. 59-178625 discloses a disk wherein the surface of the recording medium substrate is coated with a hard valve metal or its nitride, and a layer of the oxide of said valve metal is then formed on the resulting surface. Further, Published Unexamined Japanese Patent Application No. 61-199232 discloses a magnetic medium comprising a non-magnetic substrate, a hardened non-magnetic layer covering its surface, a non-magnetic base layer of Si, Ti, Mo, W or Zr covering this hardened non-magnetic layer, and a layer of a magnetic medium covering this non-magnetic base layer; and Published Unexamined Japanese Patent Application No. 61-199224 discloses a magnetic medium comprising a non-magnetic substrate, a hardened non-magnetic layer covering the substrate, a non-magnetic base layer of TiO.sub.2, TiC or TiN covering this hardened non-magnetic layer, and a layer of a magnetic medium covering this non-magnetic base layer.
However, as the average grain size of titanium is normally about 50 .mu.m which is relatively large, discrepancies due to crystals arise during precision polishing or precision grinding when titanium is used as a magnetic disk substrate, and it is difficult to obtain satisfactory surface properties. In order to overcome this problem,
(1) the grain size could be made finer by controlling forming and heat treatment, or
(2) a structure could be adopted wherein a film of fine crystalline material is formed on a titanium base surface.
In the case of (1), however, a limit to grain size is reached at about 10 .mu.m, and even with a grain size of this order, fully satisfactory surface properties cannot be obtained due to difference of crystal orientation. Further, in the case of (2), various problems arise as follows.
The above-mentioned Japanese Patent No. 52-105804 is an example of (2). In this case, there are crystals with various orientations on the titanium base surface. As the rate of oxidizing or nitriding is different depending on the orientation, it is difficult to obtain a uniform oxide or nitride film which leads to poorer yields and higher manufacturing costs. It is reported that if an oxide film is formed by oxidizing on the titanium base surface to such a thickness as to obtain high level surface properties after polishing, it is necessary to form a white oxide film of thickness not less than 1500 .ANG. ("Nihon Setchaku Kyokaishi" (Journal of Japanese Adhesives Association), Vol. 21, No. 1, 1985, page 32). If however a white oxide film is formed on titanium, it peels off easily ("Titanium-Zirconium", Vol. 32, No. 1, Jan. 1984, page 19), hence yields decline and manufacturing costs increase. If on the other hand a nitride film is formed by nitriding on the titanium base surface to a thickness required to obtain high level surface properties after polishing, cracks appear easily in the nitride film on the surface ("Nihon Kinzoku Gakkaishi" (see Materials Transactions Japan Institute Metals), 1966, Vol. 30, photo on page 28), yields still decline and manufacturing costs increase.
The technique disclosed in the above-mentioned Published Patent Application No. 63-142521 involves a large number of manufacturing processes, so that manufacturing costs are high and manufacturing time is long. If ceramics are used for the outermost layer, adequate surface properties cannot be obtained as ceramics contain a large number of pores. In the case of glass, very high level surface properties with Ra (explained later)=approx. 0.005 .mu.m are obtained, but the heads stick to the disk surface which is undesirable. Further, in the case of a glass surface, impurities in the glass such as Na and Ca diffuse into the magnetic film with rise of temperature when the magnetic film is formed, and these have an adverse effect on the magnetic properties of the magnetic recording medium.
Further, in the techniques disclosed in above-mentioned Published Unexamined Japanese Patent Applications Nos. 59-178625, 61-199232 and 61-199224, the base is merely coated with a layer of metal or its oxide, nitride or carbide, and surface properties after polishing cannot be said to be satisfactory.
Even with magnetic disk substrates having titanium as their principal constituent, therefore, substrates with satisfactory properties have still not been obtained.